Conventional coils used in moving-coil type phonograph pickups (cartridges) for picking up audio signals from phonograph records (discs) comprises at least one coil which has a winding made of a conductive wire. It is generally known that a coil which is light in weight is advantageous since lighter coils have less influence on the vibrations of the vibratory member, such as a stylus arm, to which the coil is secured. In order to provide a light coil a fabricating method of an IC (integrated circuit) was recently adopted and a coil which is made of a microstrip of spiral pattern was developed. This newly developed coil comprises a conductive spiral pattern layer formed on a suitable substrate and is produced by well known photolithographic processes.
Although the above described newly developed coil is superior and is advantageous in that the weight of the coil proper is remarkably reduced compared to the conventional winding type coils, the output voltage of the spiral pattern type coil is in the same level as that of the conventional winding type coils. As is well known, the output voltage of a moving-coil type phonograph pick up is much lower than that of the moving magnet type pickups. Therefore, when a moving-coil type phonograph pickup is employed, a step up transformer is required to raise the output voltage to a sufficient level prior to feeding the output to the input of a preamplifier.
Provision of a step up transformer causes the input signal of the preamplifier to be deteriorated in the signal to noise ratio. Therefore, it would be advantageous, in view of high fidelity sound reproduction, if such a step by transformer were omitted. In order to directly apply the output voltage of the coil of a dynamic phonograph pickup to a preamplifier by omitting a step up transformer, the output voltage of the coil has to be high enough so as to meet the requirement of the preamplifier.
Since the output voltage of a coil is in proportion to the number of turns of the winding or conductive element, the number of turns has to be increased to generate a high voltage. However, it is impossible to increase the number of turns when a conventional winding type coil is used inasmuch as the increase in the number of turns directly results in the increase in weight deteriorating the frequency characteristic of the pickup. When a spiral pattern type coil is used, the number of turns may be readily increased without raising such a problem in connection with weight since, a spiral pattern type coil is so light that it widely differs from that of a conventional winding type coil.
However, the increase in the number of turns of a spiral pattern type coil requires the increase in size, such as the diameter, of the coil unless the density of the spiral pattern is increased. The density, i.e. the number of turns per a given unit area, cannot be increased due to the manufacturing limit defined by the nowaday technique of photolithographic process. Therefore one possible way for increasing the number of turns, which has been taken into account hitherto, is to increase the diameter of the spiral pattern. However, in case of using a large diameter spiral pattern coil, the plane of the spiral pattern is apt to undulate in receipt of vibrations, resulting in the deterioration of the frequency characteristics of the output signal. Therefore, this method of increasing the diameter of the spiral pattern for having a large number of turns is not also practical.